PARAFLUVIAL NITROGEN DYNAMICS IN A DESERT STREAM ECOSYSTEM

We investigated nitrogen dynamics over a 15-mo period in the parafluvi al zone (the part of the active channel without surface water) of Syca more Creek, Arizona, a nitrogen-limited Sonoran Desert stream. The par afluvial zone and surface stream are linked hydrologically; thus, nitr ogen dynamics in the parafluvial zone potentially influence whole-syst em functioning. We identified discrete parafluvial flowpaths by follow ing the movement of fluorescent dye through gravel bars over time, sam pled subsurface water along these flowpaths, and collected parafluvial sediments for measurement of nitrification rate. Water samples were a nalyzed for nitrate-N, ammonium-N, dissolved oxygen, temperature, and conductivity. Nitrate-N concentration increased along parafluvial flow paths, with the largest increases occurring in summer. Although ammoni um-N concentration was low and did not vary with season or location on flowpath, dissolved oxygen declined as water moved through parafluvia l gravel bars. Net nitrification rate was highest in the summer and at the heads of flowpaths where surface water entered the parafluvial zo ne, suggesting that nitrification may be dependent upon ammonium, diss olved organic nitrogen, or particulate organic nitrogen imported from the surface stream. Overall, the parafluvial zone of Sycamore Creek wa s a source of nitrate to the nitrogen-limited surface stream, and may play an important role in the productivity of the stream ecosystem.